1. Field of the Invention
The present invention relates to a circuit arrangement for the production of a control signal, in particular for level regulation, in a receiving channel which is subject to interference. More particularly, the invention relates to such an arrangement wherein an amplitude-dependent control signal is derived from the phase coded input signal, wherein, in the receiving channel, a frequency regulating circuit contains an oscillator which is synchronized by means of the received signal, wherein the received signal is superheterodyned with the output signal from the oscillator in two mixers, on the one hand without delay and on the other hand with 90.degree. delay, and wherein the mixed products are fed through low-pass filters to derive signals which indicate the frequency shift between the input signal frequency and the oscillator frequency, which signals, having been converted again in a third mixer, serve as a control value for the tuning of the oscillator.
2. Description of the Prior Art
For optimum signal processing, it is frequently desirable for the useful amplitude level in a receiving channel, which has interference superimposed thereon, to exhibit a specific value. This necessitates a control signal which acts on a corresponding adjusting element, e.g. a regulated amplifier, an adjustable threshold or the like. This applies, in particular, to radio receivers and especially to circuit arrangements which operate with carrier reconstruction of a phase coded digital signal. The control signal is to depend only upon the useful power of the received signal, and superimposed interference, e.g. noise or other interference, and likewise the superimposed information, are not to appear in the control signal.
There is a series of possibilities of constructing circuits for the production of the control signal in receiving channels of the type mentioned above which are subject to interference. However, these always involve the disadvantages which have been set forth in detail below in connection with FIG. 1. To summarize, however, it may be said that the known circuits either inadequately suppress the superimposed noise interference during the formation of the control signal, or that synchronization errors in the phase control loop influence the production of the control signal for the level regulation.